This invention relates generally to thermoplastic composites and more particularly concerns threaded end connections for thermoplastic composite rods.
Thermoplastic composites consist of fiber reinforcement embedded in a matrix material. In particular, in pultruded thermoplastic rods, the fiber reinforcement is aligned with the longitudinal axis of the rod. Unidirectional fiber composite rods can also be produced by several other processes including compression molding, thermoforming, etc. Rods made by these processes exhibit very favorable characteristics for strength, weight, fatigue resistance, thermal expansion, and thermal and electric conductivity, all in a medium permitting the reforming of the rod by reheating, bending to a desired shape and cooling.
However, when thermoplastic composite rods are used in applications requiring threaded ends, minimal application of torque or tension at the threaded end results in a failure of the threads and therefore of the rod.
There is at present no method known in the art for producing a thermoplastic pultruded rod with a threaded end able to withstand significant torque and tension.
The present inventors have themselves developed and tested a number of potential solutions to the problem which have produced unsatisfactory results. For example, they have reheated the ends of pultruded thermoplastic composite rods to temperatures above melt temperature of the rod and then compressed the end of the rod in a cold mold to obtain the desired threaded configuration. It was hoped that in this reformation process, the unidirectional fibers would be rearranged to provide strength to the threads. However, the resulting threaded end exhibited a core having high fiber content and threads having a high resin content, resulting in a lack of shear strength in the threads. Application of a tensile force to the threads therefore resulted in a stripping of the high resin threads away from the high fiber rod. The result was a threaded end having virtually no shear strength whatsoever.
In another attempt of solving the problem, the inventors shaved a portion of the end of a rod to produce an end having slightly less diameter than the main portion of the rod. This shaved end was then heated to above melt temperatures and tightly wrapped with a thermoplastic composite tape simultaneously heated to above melt temperatures. It was hoped that the helical relationship of the unidirectional fibers of the tape in relation to the axial unidirectional fibers of the rod would eliminate shear problems resulting from a lack of fiber in the threads. However, it was found that only the flow of resin causing the tape resin and rod resin to bond together held the tape to the rod. As a result, while this attempt provided a threaded end displaying better tensile strength, there was no significant improvement and the threads again sheared from the rod.
The inventors also attempted to cut threads into the end of a rod with a die, thus maintaining fiber within the threads, the fibers then being cut at the exterior surfaces of the threads. The resulting thread was then heated to above melt temperature and compressed to seal the cut surfaces. This was found to provide improved tensile strength, but, probably because the threads were cut, the threaded end was unable to withstand any significant amount of torque.
It is therefore, an object of the present invention, to provide a pultruded thermoplastic composite rod having a threaded end capable of withstanding high torque and tensile forces as well as to provide a method by which threaded ends may be post-formed in any thermoplastic composite rod.